Proteins are tenaciously bound to messenger ribonucleic acid (mRNA) in the cytoplasm of mammalian cells forming messenger ribonucleoprotein (mRNP) complexes. These mRNP particles occur unattached to ribosomes in cytoplasmic pools and engaged in translation in polyribosomes. Polysomal mRNP can be released as free particles by treatment with physical or chemical disruptive agents or as a consequence of translational level inhibition of protein synthesis. Murine sarcoma 180 (S-180) tumor cells reversibly accumulate cytoplasmic pools of inactive mRNP when protein synthesis is repressed at an initiation step by amino acid deprivation. The objective of the proposed study is to identify and characterize the protein components of S-180 mRNP and determine their possible role in translation level control of protein synthesis. Oligo(dT)-cellulose affinity chromatography and CsSO4 equilibrium density gradient centrifugation will primarily be used to isolate and characterize S-180 mRNP particles. Specific experimentation is designed to 1) identify and compare, by SDS-polyacrylamide gel electrophoresis, proteins associated with mRNA from S-180 cells actively engaged in protein synthesis and those which are repressed in translation by amino acid deprivation, high temperature shock, and arrest in mitosis; 2) classify mRNP proteins according to their association with the non-poly(A) or the poly(A) region of S-180 mRNA by specific ribonuclease elution of proteins from mRNP immobilized on poly(U)-sepharose columns, and 3) determine the arrangements of proteins on S-180 mRNA by limited fragmentation of mRNP.